Polyester resins are widely used as coating materials for metal sheets for the purpose of corrosion inhibition, because of their excellent mechanical properties, electrical properties, heat resistance, gas barrier properties and adhesion with metals.
However, since the metal adhesion, impact strength and gas barrier properties of a polyester resin are strongly dependent on its degree of crystallization, the desired properties cannot be achieved without strict control of the crystal structure inside the coating. Specifically, it is necessary to reduce the crystallization degree at the resin interface that contacts the metal in order to achieve good adhesion, while increasing the crystallization at the other sections in order to ensure impact strength and gas barrier properties; it has therefore been necessary to provide a suitable gradient of crystallization inside the coating that can simultaneously satisfy the adhesion, impact strength and gas barrier properties. As a result, the conditions for the lamination step have been severely restricted.
As one means of overcoming these disadvantages of polyester resins, Japanese Unexamined Patent Publication HEI No. 3-269074 has disclosed a method of laminating a resin composition comprising a crystalline polyester resin and an amorphous polyester resin. While this method allows improved adhesion since the degree of crystallization can be easily lowered at the interface during the lamination step, it also reduces the gas barrier properties and impact strength, and the expression of both properties has required restrictions in the steps, such as using a biaxially stretched film to actively retain crystallization.
Also, Japanese Unexamined Patent Publication HEI No. 7-195617 discloses a technique for lamination of a metal sheet with a coating comprising a composition that contains a polyester resin and an ionomer resin. Since this technique can maintain impact strength even with a lower degree of crystallization, it can provide both adhesion and impact strength; however, it has not been able to achieve sufficient improvement in impact strength at low temperatures.
Furthermore, Japanese Unexamined Patent Publication HEI No. 7-290643 and Japanese Unexamined Patent Publication HEI No. 7-290644 disclose techniques wherein a three-component composition of a polyester resin, ionomer resin and polyester elastomer is used as the coating for a metal sheet. These techniques, however, while providing some improvement in impact strength at low temperatures and impact strength at room temperature, have not allowed improvement to a sufficient level.
In Japanese Unexamined Patent Publication SHO No. 58-17148 there is disclosed a polyester composition containing an aromatic polyester with a specific glycidyl group-containing copolymer and a specific ethylenic copolymer. However, this patent does not disclose or suggest a resin coating structure, forming the basis of the present invention, whereby an elastomer resin capsulated by a polar group-containing resin is dispersed in a polyester resin. Moreover, the composition disclosed in this patent is a resin composition used to obtain molds by injection molding or extrusion molding, and therefore differs substantially from the metal sheet coating resin film of the present invention in terms of its purpose of use.
It is therefore an object of the present invention to provide a resin composition for coating a metal sheet that exhibits not only excellent impact strength, chemical resistance, moldability, heat resistance and gas barrier properties, but also excellent adhesion with metals.
It is another object of the invention to provide a resin film for coating a metal sheet employing the resin composition, to provide a resin-coated metal sheet coated with a resin coating of the laminated resin film, and to provide a resin-coated metal container formed by molding the resin-coated metal sheet.